12 x 400m @ 5k pace or faster; Recovery is the short jog, across the field back to the start.
(Maybe run 6 of these at 5k pace, then the last 6, drop the pace a few seconds).
Dessert? After 2-3 min. jog, run 1x 800m, first 400 @ 5k, then kick in the last 400m.